In theory, an oscillator comprises a resonant circuit, i.e. usually a resonant circuit formed by a parallel or series connected inductance (L) and capacitance (C). For its oscillation, two conditions have to be met: the sum of the amplification and losses of the circuit must be zero and the closing the loop must invert the phase by 180 degrees. In practice, an oscillator can be implemented, for instance, with reactive components forming an LC resonant circuit, said components determining the oscillation frequency, and with negative resistance annulling the resistive losses thereof, said resistance being formed with a transistor and a feedback capacitor.
A resonant circuit may also be implemented using a crystal oscillator in which the resonant frequency is determined on the basis of the piezoelectric properties. A directly feedback, phase-inverting amplifier, i.e. ring oscillator may also be used. The oscillation frequency of a reactive oscillator is f.sub.res =1/(2.pi..sqroot.LC), so that the oscillation frequency can be controlled by changing the capacitance of the circuit, e.g. using a capacitance diode, i.e. vatactor, in the resonant circuit. It is well known that the capacitance thereof is dependent on the value of the reversed voltage affecting there through. When a feedback amplifier is in question, the delay of the amplifier can be controlled, said change causing a change in the oscillation frequency.
Voltage controlled oscillators (VCO) are particularly well suited for use in a phase-locked loop as mentioned above. Their use is therefore common in multi-channel radio apparatus frequency synthesizers because therewith it is convenient to generate different frequencies as required. In a phase-locked loop the voltage obtained from the phase comparator of the loop forms the control voltage of the oscillator. State of the art oscillators have been implemented with discrete components, so that separate components have to be used in all applications employing a phase-locked loop. The oscillator is, in addition to the loop filter, an obstacle to the complete integration of the phase-locked loop. The number of discrete components can be somewhat reduced by employing commercially available, prefabricated, plastic-encapsulated integrated VCO circuits. However, they require an external, a so-called tank circuit, charging and discharging which the oscillation is based on. The tank circuit consists of an inductance, a capacitor and a capacitance diode, which cannot have been totally integrated on silica, so that no one has managed to build a complete oscillator implemented in the form of an IC circuit.
The above oscillator circuits known in the art are encumbered with certain drawbacks. The oscillator is highly sensitive to disturbances and in implementing it, especially in RF applications, particular attention has to be paid to protection against disturbances caused by electromagnetic interference (EMI). In direct frequency modulation in which the modulating signal is directly summed with the control voltage of the voltage controlled oscillator, the modulating signal must be filtered and its level has to be maintained sufficiently low. The aim of said operations is to improve the signal/noise ratio of the oscillator. In radio phone applications of the oscillator, the discrete components employed constitute a restriction to complete integration of the voltage controlled oscillator, and consequently, that of the synthesizers. In integrating PLL circuits, creating the requisite high filtering time constants is also difficult to accomplish. A drawback related to oscillators known in the art is also that the frequency range within which an oscillator can be controlled is rather narrow. It would be highly advantageous if such wide frequency range could be provided for in one and same oscillator that the same oscillator could be used in different applications. As to radio phones, this means that in different telephone versions one oscillator could be used instead of version-specific oscillators, as is common practice nowadays. With the means currently used such an oscillator can be constructed in which the frequency range is very wide but then the VCO coefficient (frequency/voltage) has to be great. In such instance, the signal/noise ratio is also small so that the oscillator is readily modulated by noise and various interference signals, and a sufficient frequency standard of purity is not reached. Typically, the frequency of the oscillators known in the art can be controlled only in the range 20 to 60 MHz.